Conventionally, a lamp light source has been mainly used as a light source of a projection-type image display apparatus. However, the lamp light source has a defect that it has a small quantity of red light, a short lifespan, and so on. Thus, in recent years, in place of the lamp light source, a surface-emitting light source such as a light emitting diode (LED) which has a lifespan longer than that of the lamp light source tends to be used. Since light emitted from the LED has a narrow wavelength range, the light source of the projection-type image display apparatus can achieve a wide color reproduction range by combining LEDs of red (R), green (G) and blue (B) colors.
For example, Patent Document 1 proposes an illumination system which uses LEDs of R, G, B colors and an integrator rod. In this system, respective colors of light beams emitted from the LEDs of the respective colors are collimated by collimate lenses corresponding to the respective colors, the collimated light beams are synthesized by a dichroic mirror or the like, and the synthesized light is focused on an incident surface of the integrator rod by a common condenser lens.